1. Field of the Invention
The present invention relates to a liquid crystal display panel driving apparatus for driving a color liquid crystal panel having color filters provided between scan electrodes and signal electrodes.
2. Description of the Related Art
A color liquid crystal panel for use in liquid crystal color televisions, etc. displays an image in full color by a combination of red (R), green (G) and blue (B) pixels. In displaying a color image by a combination of red, green and blue pixels, display of a full color image with high reproducibility requires that the luminances of these three types of pixels be balanced. Due to the wavelength dependency of a liquid crystal element, however, this property influences the color reproducibility of display pixels. This short-coming is more prominent for a color liquid crystal display element having its view angle characteristic improved by reducing the cell gap (the thickness of a liquid crystal layer).
In other words, to improve the view angle characteristic of a liquid crystal display element, the value of the liquid crystal element, .DELTA.n.multidot.d (.DELTA.n: birefringence and d: cell gap) should be set small. This is because a liquid crystal element having a large .DELTA.n.multidot.d has a large change in this value depending on the view angle (viewing direction). Since the value of .DELTA.n.multidot.d greatly changes depending on the view angle means that a large change in contrast occurs depending on the view angle. Therefore, the change in contrast which is depended on the view angle can be reduced by setting the cell gap of the liquid crystal element small to thereby make .DELTA.n.multidot.d small. According to the color liquid crystal element, however, reduction in .DELTA.n.multidot.d increases the wavelength dependency of transmitted light, as shown in FIG. 1. FIG. 1 illustrates the relation between .DELTA.n.multidot.d (.mu.m) and an amount of transmitted light (%), where "R" is a characteristic of 650-nm (red) wavelength light, "G" is a characteristic of 550-nm (green) wavelength light, and "B" is a characteristic of a 450-nm (blue) wavelength light. The presence of the aforesaid "wavelength dependency" means that the light transmittivity varies from one wavelength to another and that, in view of three-color (R, G and B) lights, the lights with different wavelengths which pass the respective filters have different intensities. This is because that the characteristic showing a change in transmittivity with respect to .DELTA.n.multidot.d differs from one light having a specific wavelength to one of the three colors to another; the greater the wavelength dependency of transmitted light, the lower the color reproducibility of a displayed image.
As a conventional solution to the above short-coming, the thicknesses of the individual color filters FR, FG and FB are set different from one another to adjust the cell gaps dR, dG and dB for pixel display sections for these colors, as shown in FIG. 2, whereby the transmittivities of the individual color lights are balanced by changing the values of .DELTA.n.multidot.d of the individual pixel display sections. FIG. 2 illustrates the cross section of the configuration of a color liquid crystal display element. Referring to this diagram, reference numerals 1 and 2 denote a pair of upper and lower transparent substrates (glass plates) facing each other with a liquid crystal layer 3 in between, and these substrates are adhered through a frame-shaped seal member (not shown). A number of parallel transparent scan electrodes 4 are arranged in stripe form on the inner wall of the upper substrate 1 (which faces the lower substrate 2) in the horizontal direction in the diagram. A number of parallel transparent R, G and B signal electrodes 5a, 5b and 5c are arranged on the inner wall of the lower substrate 1 (which faces the upper substrate 1) in such a direction as to cross the scan electrodes 4. These signal electrodes 5a-5c are respectively provided on the striped color filters FR, FG and FB, which are provided on the surface of the lower substrate 2 in such a way as to cross and face the individual scan electrodes on the upper substrate 1. FR is a red filter, FG a green filter, and FB a blue filter, and these color filters FR, FG and FB are alternately arranged, as illustrated in FIG. 2. The color filters FR, FG and FB have their surfaces covered with a transparent insulative film 6 on which the signal electrodes 5a, 5b and 5c are formed. The end portions of the scan electrodes 4 and signal electrodes 5a, 5b and 5c are extracted as driver connection terminals to side edge portions of the substrates, as shown in FIG. 3. The scan electrodes 4 are supplied with scan signals X1, X2, . . . , and the signal electrodes 5a, 5b and 5c are supplied with R, G and B image signals in association with the respective color filters FR, FG and FB. Referring to FIG. 2, reference numerals 7a and 7b are orientation process films provided on the electrode-forming surfaces of the substrates 1 and 2, and 8a and 8b are deflection plates.
With the above arrangement, a full color image with a high reproducibility can be displayed with balanced luminances of the red, green and blue pixels by changing the thicknesses of the individual filters FR, FG and FB to balance their transmittivities. In this case, with the same .DELTA.n.multidot.d for the individual pixel display sections, the red light has the highest transmittivity among the three color lights, and the green light has the second highest transmittivity, with the blue light having the lowest one. To balance the transmittivities of the individual color lights, therefore, the red filter FR for passing red light having the highest transmittivity is made thinnest, the green filter FG thicker and the blue filter FB thickest, as shown in FIG. 2. In general, therefore, there is a height difference of about 1 .mu.m between the red filter FR and green filter FG and about 2 .mu.m between the red filter FR and blue filter FB.
As described above, the conventional liquid crystal display element has a large height difference between the red filter FR and blue filter FB, so that the orientation process films 7a and 7b and transparent insulative film 6 may not be formed evenly or the insulative film 6 is more likely to be cut by the etching process for forming the transparent electrodes 4.